Almost zero chance that the rear main is leaking, I made a post on it a while back as to why, it was a crucifix seal discussion. It could be the crucifix seals, but my guess is the truck went to the dealer a long time ago for the leak and they already replaced those. The other possibility is the rear cam plug, I have pulled two engines that showed evidence of leakage at that point. Only way to see it is to pull the flywheel.